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80 Instruction-Decoding Optimizations Chapter 4
25112 Rev. 3.06 September 2005
Software Optimization Guide for AMD64 Processors
4.6 Short Instruction Encodings
Optimization
Use instruction forms with shorter encodings rather than those with longer encodings. For example,
use 8-bit displacements instead of 32-bit displacements, and use the single-byte form of simple
integer instructions instead of the 2-byte opcode-ModRM form.
Application
This optimization applies to:
• 32-bit software
• 64-bit software
Rationale
Using shorter instructions increases the number of instructions that can fit into the L1 instruction
cache and increases the average decode rate.
Example
Avoid the use of instructions with longer encodings, such as those shown here:
81 C0 78 56 34 12 add eax, 12345678h ; 2-byte opcode form (with ModRM)
81 C3 FB FF FF FF add ebx, -5 ; 32-bit immediate value
0F 84 05 00 00 00 jz label1 ; 2-byte opcode, 32-bit immediate value
Instead, choose instructions with shorter encodings, like these:
05 78 56 34 12 add eax, 12345678h ; 1-byte opcode form
83 C3 FB add ebx, -5 ; 8-bit sign-extended immediate value
74 05 jz label1 ; 1-byte opcode, 8-bit immediate value